Voice-frequency signal receiver with speech-immunity circuit



March 22, 1966 BEYERLE 3,242,267

VOICE-FREQUENCY SIGNAL RECEIVER WITH SPEECH-IMMUNITY CIRCUIT Filed Nov.13, 1962 PM'VENTOR BEYERLE United States Patent 3,242,267VOICE-FREQUENCY SIGNAL RECEIVER WITH SPEECH-lMlVIUNITY CIRCUIT ErnstBeyerle, Fellbach, Wurttemberg, Germany, assignor to InternationalStandard Electric Corporation, New

York, N .Y., a corporation of Delaware Filed Nov. 13, 1962, Ser. No.237,181 Claims priority, application Germany, Nov. 16, 1961, St 18,569 7Claims. (Cl. 179-84) The invention relates to a V.-F. signal receiverwith speech-immunity circuit in telecommunication, particularlytelephone systems.

At the transmission of signals within the voice-frequency range,particularly of V.-F. ringing, dialling-, and control-signals over tolllines in telecommunication systems with amplifiers, it is necessary tohave the receivers provided for these signals constantly connected tothe transmission path. It is therefore required to block these receiversagainst speech frequencies or noise voltages, because the speechfrequencies are transmitted on the same transmission path.

In order to obtain suitable protection against wrong signals caused bythe speech, the V.-F. receivers are equipped with speech-immunitycircuits designed in such a Way that only the signal receiver operates,if the signal frequency solely arrives over the transmission path. Allfrequencies outside the signal-frequency range block the signalreceiver. Even if the signal frequency is included in the speechfrequencies, the blocking is still effective so that the signal receiverdoes not furnish a faulty output signal. For all of these signalreceiving methods it is necessary that on the transmitting end speechtransmission and signal transmission are time separated.

Various speech-immunity circuits are known. In some V.-F. receiversknown a parallel-resonant circuit tuned to the signal frequency, isinserted into the plate circuit of a preordinated amplifier tube andseries-connected with a series-resonant circuit, tuned to the samefrequency. The voltages occurring in the circuits are rectified and ledto two separate relays, one serving as a receiving relay, the other oneas a speech-immunity relay.

In another solution of the problem the rectified voltages are led to acommon receiving relay having two opposite windings. This relay onlyoperates if the 10c:- ing circuit does not furnish any output voltage,that is, if only the signal frequency is applied to the parallelresonantcircuit.

These well known arrangements require a large expenditure for filters toseparate the signal frequency and speech frequencies. Even with the manyfilter arrangements they still exhibit a very high speech sensitivity.

Other voice-frequency signal receivers are known Which minimize pulsedistortions. Limiter circuits and regulating devices are used with thesereceivers to insure that a constant voltage is provided at the filterinput independent of the receiving amplitude at the signal-receiverinput. By this method the filter circuits may be simplified.

In order to keep the expense of filters in signal receivers as small aspossible and to none the less receive a non-distorted output signal withspeech imunity, another known arrangement separates the frequenciesacross a bridge circuit. One arm of the bridge is formed by aseries-resonance circuit tuned to the signal frequency. From thiscomponent or branch the signal frequency is derived. The blockingvoltage is tapped from the zeropoint branch of the bridge.

Such bridge circuit arrangements are advantageous in that the range ofthe signal frequency required is very small, and that frequencies nearthe signal-frequency range already cause extensive blocking. The highspeech 3,242,267 Patented Mar. 22, 1966 immunity of such circuits ishowever, obtained only by the use of an expensive bridge circuit,particularly the transformers required to derive the signal frequency.Simplification of the bridge circuit, such as by converting to aresistance bridge, causes high attenuation which reduces the earresponding sensitivity of the signal receiver. Thus, it means moreamplifiers are required.

Therefore, an object of this invention is to provide new, efficient andunique voice-frequency receivers with speech immunity.

Another object of this invention is to provide voice frequency receiversthat separate the control signal frequencies with a minimum ofequipment.

A further object of this invention is to provide voice frequencyreceivers employing series resonant circuits that use mutual inductancein the tank to provide a narrow bandwidth and a high degree of voiceimmunization.

In accordance with a preferred embodiment of the voice frequencyreceiver of this invention all frequencies, speech and signal, occurringat the output of a non-selective preamplifier, are :connected to aseries-resonant circuit formed by the mutual inductance of a transformerand a capacitor. The resonant circuit is tuned to the signal frequency.The signal frequency is connected over an additional winding of thetransformer to control the receiving switching means. Since theseries-resonant circuit is of 'high quality a narrow signal-frequencyrange results therefrom. Obtaining the signal frequency over theadditional winding of the transformer permits the optimum matching ofthe receiving switch ng means to the series-resonance circuit. Aseparate output from the series-resonance circuit is used to provide theblocking voltages. Derivation of the voltages is selected in such a Waythat rectified signal voltage and rectified blocking voltage occurringacross the series-resonance circuit are series-connected in oppositedirections in the control circuit of the receiving-switching means.According to a further development of the signal receiver in compliancewith the invention several series-resonance circuits are provided at theoutput of the preamplifier, these circuits are tuned to differentfrequencies and connected to the pertinent receiving-circuit elements.It is possible thereby to obtain in a simple manner, a multi-frequencyreceiver in which the individual signal-frequency receiving circuitshave separate blocking circuits. The correct functioning of themulti-frequency receiver requires that the code signals be selected insuch a way that only one of the connected one-frequency receivers isoperated at a time. The signal receiver, according to the invention,provides a pro-amplifier that is equipped with an amplitude limiter oris an AGC pre-amplifier which furnishes a constant output signal for allinput signals Within a certain amplitude range.

The V.-F. signal receiver will be explained in greater detail withreference to the single drawing.

Means are provided for amplifying the input signals. The amplifier shownin the drawing has two stages. The active element in each stage is a PNPtransistor Q1 and Q2 respectively. The first stage amplifier is coupledto the transmission line through its input terminal 1, 2. The base oftransistor Q1 is connected to terminal 1 through coupling capacitor C1.Biasing is obtained over the Voltage divider comprising R1, R2. The baseof transistor Q1 is coupled to the point of coupling of the tworesistors R1, R2. The other side of R1 is connected to the positivevoltage source while the other side of resistor R2 is connected toterminal 2 which is grounded.

The emitter of transistor Q1 is connected to the positive voltage supplythrough bias resistors R3, R4. Filter capacitor C2 shunts resistor R4 tostabilize the first stage.

The collector of transistor Q1 is connected to ground through loadresistor R6. The base of transistor Q2 is also connected to thecollector of transistor Q1.

The base of transistor Q2 is connected to positive voltage through biasresistor R7. The bias current thus flows from positive voltage throughresistors R7, R6 to ground. The collector of transistor Q2 is connectedto positive voltage through series bias resistors R8, R9. Filtercapacitor C3 shunts resistor R9. The load for transistor Q2 istransformer T1.

Means are provided for obtaining a constant voltage output from the twostage amplifier. In greater detail, a limiting circuit such as thatcomprising diodes D1, D2 and capacitor C4 is connected to the couplingpoint of transistors Q1 and Q2.

Means, such as transformer T2 in series with capacitor C6, are providedfor separating out the control signals from the received signals. Ingreater detail, transformer T2 is comprised of three windings. Themutual inductance of windings I, II in combination with the capacitanceof C6 resonates at a frequency equivalent to the control signalfrequency. Thus, when a voltage output Va from the two stage amplifieris connected across the series combination of winding I of transformerT2 and capacitor C6. The voltages with the control signal firequencyflow through the resonant circuit and are thus elfectively' shorted.However, a voltage is induced in winding III by the control signalvoltages in winding II.

Means are provided for rectifying the output of transformer T2; such asa full wave bridge rectifier across winding H and a full wave rectifieracross winding III. The full wave bridge rectifier comprises diodesD3-D6 and filter capacitor C6. The full wave rectifier comprises diodesD8, D9 and filter capacitor C7.

Means are provided for algebraically adding the rectified signalsobtained from the control signals and speech signals and amplifying thissum. In greater detail, circuitry such as PNP transistor Q3 andassociated components is provided. The base of transistor Q3 isconnected directly to the positive output portion of the rectifyingbridge and to ground through resistors R11, R12. The negative voltageoutput of the bridge and the full wave rectifier is connected to thejunction of resistors R11, R12. The emitter of transistor Q3 is biasedby coupling to the wiper of potentiometer P1. 7 Potentiometer P1 isconnected in series wth resistor R13 between ground and positivepotential. The output at the collector of transistor Q3 controls areceiving element such as a Schmitt trigger circuit. A feedback filtercapacitor C9 connects the emitter and collector of transistor Q3.

The Schmitt trigger circuit shown comprises transistors Q4, Q5 whichonly con-duct alternately. The base of transistor 5 is connected to thenegative potential source through variable resistor R16 in series withresistor R17. Bias resistor R18 bridges both resistor R16, R17. Theemitter of transistor Q4 is connected to ground through bias resistorR17. Load resistor R20 connects its collector to negative voltage.Transistor Q4 is coupled to transistor Q5 through resistor R21 bridgedby capacitor C10.

The base of transistor Q5 is biased by resistor R22 connected to thejunction point of resistor R19 and the emitter of Q4 as is well knownpractice with this type of trigger circuit. The collector of transistorQ3 is connected to negative potential through resistor R23 and it isalso connected directly to output terminal A. Thus, normally transistorQ4 conducts and output A is negative. When transistor Q3 conducts,transistor Q4 is turned off and transistor Q5 conducts effectivelyplacing ground at output terminal A. Transistor Q3 conducts only whenspeech signals are not present along with the control signals and thusthe control signals are not blocked.

The operation of the circuit will now be explained. The voltagesarriving over the transmission path are connected across the input (E-O)of the signal receiver. The voltages are amplified to a requiredamplitude via a pre-amplifier consisting of the transistors Q1 and Q2. Alimiter circuit is provided between the first and the second amplifierstage in order to obtain a constant output voltage Va from thepre-amplifier over the entire receiving-level range. This circuit is aseries-connected circuit well known to those skilled in the art, namelya capacitor C4 and the parallel-connected diodes D1 and D2 operating inopposite directions.

The output voltage Va is connected to the series-resonant circuit tunedto the signal frequency. In order to give this circuit high quality, theresonant-circuit inductance is formed by the mutual inductance of thetwo symrnet-rical windings I and H of the transformer T2. Thus, theresonant circuit frequency curve is very steep and only a narrowsignal-frequency range will pass through winding III. All otherfrequencies are normally restricted to the output of the resonantcircuit (winding II and capacitor C). Only if the signal frequencyarrives alone will the rectified voltage derived from winding III switchthe Schmitt-trigger that is used as a receiving element with thetransistors Q4 and Q5. At the output, A, resistance ground potential isnormally found. If, however, speech is transmitted on the path, thespeech-immunity circuit also furnishes an output voltage which cancelsthe control voltage furnished by the signal circuit. Both voltages areseries-connected in opposite direction in the control circuit oftransistor Q3. The amplified output voltage of Q3 switches theSchmitt-trigger when the blocking speech voltage is present. The outputA then is at a negative potential.

As may further be understood from the drawing several signal-receivingcircuits tuned to different signal frequencies, may be connected to theoutput of the preamplifier. This is indicated by several lines on thesecondary side of the output transformer.

The amplifier transistor Q3 serves to increase the control and theblocking voltage. Instead of the limiter circuit in the pre-amplifieralso a pre-amplifier with AGC may be used in order to keep the outputvoltage Va constant. Thereby only a small portion of the resonance curveof the series-resonance will be used and therefrom results a very smallsignal-frequency range independent of the signal level.

While the principles of the invention have been described above inconnection with specific apparatus and applications, it is to beunderstood that this description is made only by Way of example and notas a limitation on the scope of the invention.

What is claimed is:

1. A voice frequency signalling system comprising means for receivingvoice frequency signals that include speech signals and control signals,said control signals employing a pre-determined portion of the entirevoice frequency range, said receiving means comprising selective circuitmeans including series-resonant circuit means comprising mutualinductance for separating said control signals from said speech signals,trigger means having a normal condition and an operated condition,trigger control means inductively coupled to said selective circuitmeans responsive to said received voice frequency signals includingcontrol signals, and summing means: included in said trigger controlmeans acting responsive to said received voice frequency signalsincluding only control signals for operating said trigger means to said.operated condition and acting responsive to said received voicefrequency signals including speech signals for blocking said triggercontrol means to operate said trigger means to said normal condition.

2. In the voice frequency signalling system of claim 1 wherein saidselective circuit means comprises nonselective pre-amplifier meansproviding a constant amplitude output signal.

3. In the voice frequency signalling system of claim 1 wherein saidselective circuit means comprises non;

selective pre-amplifier means having amplitude limited outputs.

4. In the voice frequency signalling system of claim 2 wherein saidselective circuit means comprises series resonant filter means,transformer means for coupling said pre-amplifier to said filter means,said filter means tuned to resonance at said control signal frequencieswith capacitor means and the mutual inductance of said transformermeans.

5. In the voice frequency signalling system of claim 4 wherein saidtransformer means comprises three winding transformer means connected togive speech signals across one of said windings in series with saidcapacitor means and control signal across another of said windings.

6. In the voice frequency signalling system of claim 5, first rectifyingmeans for rectifying said control signals, second rectifying means forrectifying said speech signals, and wherein said summing means comprisesmeans for series connecting said rectified signals for controlling saidtrigger control means.

7. In the voice frequency signalling system of claim References Cited bythe Examiner UNITED STATES PATENTS 2,686,227 8/1954 Ryall l79842,935,572 5/1960 Hastings et al l7984 2,964,650 12/1960 Radcliff et a1.l7984 3,076,059 1/1963 Meacham et al l7984 3,098,179 7/1963 Van Rossumet al. l7984 3,103,558 9/1963 Ligotky l7984 ROBERT H. ROSE, PrimaryExaminer.

WALTER L. LYNDE, Examiner.

1. A VOICE FREQUENCY SIGNALLING SYSTEM COMPRISING MEANS FOR RECEIVINGVOICE FREQUENCY SIGNALS THAT INCLUDE SPEECH SIGNALS AND CONTROL SIGNALS,SAID CONTROL SIGNALS EMPLOYING A PRE-DETERMINED PORTION OF THE ENTIREVOICE FREQUENCY RANGE, SAID RECEIVING MEANS COMPRISING SELECTIVE CIRCUITMEANS INCLUDING SERIES-RESONANT CIRCUIT MEANS COMPRISING MUTUALINDUCTANCE FOR SEPARATING SAID CONTROL SIGNAL FROM SAID SPEECH SIGNALS,TRIGGER MEANS HAVING A NORMAL CONDITION AND AN OPERATED CONDITION,TRIGGER CONTROL MEANS INDUCTIVELY COUPLED TO SAID SELECTIVE CIRCUITMEANS RESPONSIVE TO SAID RECEIVED VOICE FREQUENCY SIGNALS INCLUDINGCONTROL SIGNALS, AND SUMMING MEANS INCLUDED IN SAID TRIGGER CONTROLMEANS ACTING RESPONSIVE TO SAID RECEIVED VOICE FREQUENCY SIGNALSINCLUDING ONLY CONTROL SIGNALS FOR OPERATING SAID TRIGGER MEANS TO SAIDOPERATED CONDITION AND ACTING RESPONSIVE TO SAID RECEIVED VOICEFREQUENCY SIGNALS INCLUDING SPEECH SIGNALS FOR BLOCKING SAID TRIGGERCONTROL MEANS TO OPERATE SAID TRIGGER MEANS TO SAID NORMAL CONDITION.